GEOLOGY AND ORE DEPOSITS OF SU-AN 23 



diopside. Tlie ilvaite crystallized out earlier than the diopside did, 

 and the sulphide ores ^* still later, enclosing and filling up the 

 interstices of the diopside crystals. Consequently the gold was 

 not present at the time of the formation of the ilvaite -'. The 

 molecular rearrangement of CaCO^ in the form of marble was the 

 latest event in the formation of the contact-metamorphosed lime- 

 stone, as may be unequivocally proved by the nmtual relations of 

 these crystals. We are therefore driven to the conclusion that the 

 gold is juvenile, and must have come from deep in the interior 

 as an exudation from the eutectic mixture of the 2:ranitic masrma 

 of On-jin-san, in association with mineralizers which at first acted 

 as an acid upon the limestone and then after^vard metamorphosed 

 it into the lime-silicate-hornfels. The ore-bearing, deep contact- 

 metamorphosed zone was dismantled through denudation ^^ and a 



1) During the examination of eeveral slides of ore-rocks, it constantly recurred to my 

 mind that the paragenetic relation of component minerals shows that sulphide ores occupy 

 spots which otherwise calcite would fill. We are tlierefore driven to the conchision either 

 that the calcite and the ores settled at the same time, or that the latter ha? replaced the 

 former. The latter suggestion is the more probable, and it is moreover certain that tlie 

 replacement was not molecular or metasoinatic, but an in-filling after the calcite had been 

 removed bodily by some reagents. 



2) The general order of metallization and crystallization in our ore-deposit is ilvaite 

 diopside, garnet, sulphide ores, and the final act of metamorpliosis is the recrystallization 

 of the limestone, or is contemporaneous with the ores. 



3) It is usually supposed that contact-metamorpliic deposits can only be formed deep 

 in the zone of rock-flowage — the depth of 8-12 km beneath the surface, corresponding to a 

 pressure of about 200 atmospheres, but with a temperature below the critical points of water 

 (360°C) ; for the transportation of materials and chemical interchanges are commonly assumed 

 to take place in a liquid medium. Granting that thcoC conditions are necessary for the 

 formation of the deposits, the region under consideration must have sufiered a general denu- 

 dation of the lithosphère to the considerable depth of 4-8 miles. Chas. R. Keyes, however, 

 entertains the view that the deposits may have been formed in a shallower zone than that 

 above outlined (" Economic Geology ", vol. 3, 1909). 



Recently, Albert Bruns has made public a bold hypotheiis on the auliydrous activities 

 of volcanoes, whic'.i implies that the magma in the earth's interior is devoid of any trace of 

 water. If that be the case, not only must our theories about the deptli at which the contact- 

 metamorphic deposits are said to have been developed, be recast, but the v/hole mass of 

 hydro-igneous problems will be thrown into chaos. 



